ORGANIC EVOLUTION 



173 



m 



These nerve cells cease early to divide and develop 

 among themselves intercommunicating processes, and 

 externally, other longer and slenderer unbranched processes 

 which become the nerve fibres, and extend to remote parts 



of the body. The orig- 

 inal cell heaps become 

 the ganglia which col- 

 lectively make up the 

 cord, and each ganglion 

 becomes the centre for 

 the receipt of stimuli 

 and coordination of re- 

 sponses for all the parts 

 of its segment. These 

 cells have no other func- 

 tion than sensory com- 

 munication between the 

 parts of the body: the 

 accompanying diagram 

 (fig. 109) shows an ar- 

 rangement clearly adap- 

 ted to that office. 



Mesoderm we did not 

 find in the hydra, but in 

 the worm it gives rise to 

 the greater part of the 

 adult body. The germ 

 cells remain in the 

 mesoderm unspecialized. 

 The leucocytes, (or 

 white "corpuscles") of the blood, also, retain a singularly 

 primitive amoeboid form. They move about freely in 

 the fluids of the coelom, where they serve the useful 

 function of feeding on bacteria and other foreign substances 



n 



Fig. 109. Diagram of distribution of nerve 

 cells and fibres m the portion of the ventral 

 nerve cord of the worm lying in two seg- 

 ments (m and n) ; cell a sends fibres forward 

 and backward within the cord ; e and /, are 

 centrally located multipolar cells; all the 

 others are uninipolar; b, sends a fibre out 

 on its own side; candd, each sends a fibre 

 out too through a nerve on the opposite side 

 of the cord (after Retzius). 



